Thin film transistor (TFT) driven display device is the main stream technology in display field. A thin film field effect transistor display device typically includes a color film substrate and a thin film transistor array substrate. The thin film transistor array substrate includes a plurality of pixel units, each of which contains a thin film transistor. When a voltage applied to the gate electrode of the thin film transistor exceeds a threshold voltage value, the channel (i.e., the active layer) is turned on, connecting the source electrode and the drain electrode. The signal in the data line is transmitted from the source electrode to the drain electrode, controlling the pixel electrode connected to the drain electrode. The voltage difference between the pixel electrode and the common electrode drives the liquid crystal molecules, resulting in different rotation angles in the liquid crystal molecules and different light transmission intensity. The display device produces different images on the display panel by this mechanism. Thus, the function of the thin film transistor directly affects the display quality.